1. Field of the Invention
The invention relates to processes for connecting an optical element of a microlithographic projection exposure apparatus to a mount, and also relates to an assembly.
2. Description of the Related Art
It is known to use adhesive bonding processes to connect optical devices, such as lenses, mirrors or prisms, to metallic or ceramic mounts. However, these processes have the drawback that there is a risk of the optical device moving off-center and of stresses occurring between the optical device and the mount during the heat treatment of the adhesive. Further problems result from the low UV resistance of the adhesive and from creep and relaxation effects (setting shrinkage, post-setting effects, chemical instability) and also long-term properties that are difficult to predict.
These problems are particularly serious when mounting optical components for example in a microlithographic projection exposure apparatus, as is used for the photolithographic production of very fine structures, such as semiconductor components, and in which high-precision mounting of the optical devices with long-term stability is particularly important.
DE 197 35 760 A1 has disclosed a process for fitting optical, transparent components to metal mounts and mounted assemblies without using organic materials of this type. The transparent material, in particular quartz glass or calcium fluoride, is provided, in the region of the connection, with a coating which is able to ensure bonding between a solder alloy and the transparent material. To produce the connection, the coated transparent device and the mount, which has previously been provided with the solder alloy, are positioned on one another, jointly heated in a furnace to the melting temperature of the solder and then cooled.
U.S. Pat. No. 3,839,780 has disclosed a process for connecting metal devices, in particular aluminum devices, using intermetallic compounds formed from a first metal (mercury, gallium or indium) and a second, more noble metal, such as copper, gold, nickel, silver or palladium, which is supposed to ensure a good thermal and electrical conductivity between the metal devices. To prevent premature alloy formation from the components of the intermetallic compound, first of all a paste of these components which has solidified or “frozen” through cooling is introduced between the metal devices, and then the entire arrangement is heated in order to form the fixed bond. According to an alternative process, one of the components (e.g. comprising copper or gallium), which is in solid, pulverized form, is temporarily provided with a coating (e.g. of gallium oxide or copper oxide), which is subsequently mechanically broken open. According to a further alternative process, the two components (e.g. gallium and copper) are separately deposited, for example by electroplating, on the device surfaces to be connected, after which the device surfaces are brought into contact with one another and heated in order to form the connection.
Furthermore, U.S. Pat. No. 4,863,090 has disclosed the connection of two devices (e.g. electronic devices or optical components) by means of a process in which first of all in each case a gold layer is applied to the devices, which have been provided with a barrier layer (e.g. of nickel), and then each of the devices together with the associated layers is separately partially immersed in a mercury bath, so that a mercury/gold amalgam is in each case formed by diffusion at the respective surface. Then, the devices are joined together using the surfaces that have been coated in this way under the application of pressure.